1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing cups having a lip. More specifically, the invention relates to a method for forming a lip on a cup formed in one piece by vacuum or press molding from a thermoplastic resin foamed sheet material, such as polystyrene foamed sheet, the lip being such that it extends radially outwardly from the mouth of the cup and is bent at the end thereof radially inwardly towards the side wall of the cup, and an apparatus for forming such lip.
2. Description of the Prior Art
Thermoplastic resin foamed sheet materials have good heat-insulating properties, permit easy molding, and are highly workable into complex curved surface configurations. With such advantages, they have recently been widely used for manufacture into food and beverage containers. Indeed, cups made of such material are largely supplanting paper cups.
Generally, cups of such resin foamed material are manufactured in such a way that a piece of thermoplastic resin foamed sheet is formed into a number of cup blanks arranged in line. The individual cup blanks so formed are trimmed on the outer peripheries thereof.
Thus, each resulting cup is such that it has at its mouth portion a horizontally outwardly extending rim or such a rim having a downwardly extending portion.
In other words, the end of the trimmed rim portion is exposed outwardly of the top of the side wall. This gives the cup an unaesthetic apperance. Moreover, the outwardly extending end of the rim is easily liable to become soiled, which is undesirable from the sanitary point of view. A further disadvantage is that the cup may be easily cracked or torn at its peripheral edge. Another difficulty is that such projection feels unpleasant to the mouth.
The problem of such projection can be solved by bending a portion of the rim further against the side wall so that the end of the trimmed rim portion is not exposed outwardly. In the art of paper cup manufacturing, it is a practice to manufacture cups having a lip formed by bending the end of the rim against the side wall as described above.
In the case of cups made from a resin foamed sheet material, however, the formation of a lip involves considerable difficulty and no satisfactory lip-formation has been achieved.
For example, Japanese Patent Publication No. 38-24594 discloses a lip forming method such that the side wall of a container is heated and simultaneously pressed under rotation against a mold, whereby a lip is formed on the container. This method can be applied to containers formed of non-foamed thermoplastic resin sheet materials. In the case of a container formed of a thermoplastic resin formed sheet material, however, there are problems to be solved. One difficulty is that the material has insignificant elongation, which means poor workability for lip formation. As such, the finished product may have an uneven surface or creases, or less dimensional accuracy. Another difficulty is that because of its high heat-insulation performance such foamed sheet material can hardly be heated in a short time to such a temperature as will permit lip formation. Therefore, said method cannot be implied as it is to cups formed of foamed sheet materials.
In U.S. Pat. No. 3,291,361, it is proposed to form a lip by heating and shrinking the side wall and then bending the rim down against the inside of the container. With such simple heating as disclosed in this prior art patent, however, it is impossible to bend the rim down against the outside of the container, because such bending requiring circumferential elongation of the material. Therefore, such method cannot be employed for the purpose of forming a lip extending outwardly of the cup as intended for by the present invention.
U.S. Pat. No. 3,933,298 discloses a method such that the outer surface of a composite container body wall layer is scored by a scoring tool to define a continuous circumferential scoreline, the body wall layer being bent at the scoreline. If the container is formed of a foamed sheet material, simple scoring by a scoring tool may not achieve sufficient bending because the restoring force of the foamed sheet material will tend to restore the scorelined portion to its prior condition.